Exploring elasmobranch biodiversity in the western Mediterranean and Alboran Sea through eDNA-Based monitoring

Environmental DNA (eDNA) metabarcoding is increasingly applied for regional biodiversity assessments, yet its effectiveness for detecting mobile, low-density megafauna remains uncertain. This study employed a multi-marker eDNA metabarcoding approach (three primer pairs targeting two mitochondrial genes) across 23 sites in the Spanish Mediterranean, including the Alboran Sea, to (i) to characterise the composition of elasmobranch taxa detected by eDNA metabarcoding across sampling zones, (ii) explore spatial patterns of detection, and (iii) evaluate environmental and spatial drivers of occurrence. Ordination and permutation analyses revealed spatial structuring of overall metazoan communities, primarily driven by bathymetric gradients and proximity to marine reserves. In contrast, elasmobranch assemblages exhibited low spatial turnover. Eleven shark and ray species were detected, several of which are listed under threatened or near threatened IUCN categories, underscoring the conservation value of eDNA detections. Depth emerged as the main predictor of elasmobranch richness and composition, while read abundance showed no consistent association with environmental covariates. This work represents the first regional assessment of elasmobranch diversity in the Alboran and adjacent Mediterranean waters using eDNA metabarcoding, demonstrating its capacity to supplement traditional monitoring approaches. Overall, multi-marker eDNA effectively captures regional biodiversity gradients and detects rare, mobile taxa such as elasmobranchs. To further enhance taxonomic resolution and strengthen eDNA applications in elasmobranch conservation, we recommend three key actions: (i) increasing spatio-temporal replication, (ii) integrating targeted quantitative assays (qPCR/ddPCR), and (iii) improving reference databases.